1. Field of the Invention
The present invention relates to chemical process towers and, more particularly, to a method of and apparatus for cartridge tray sealing for chemical process towers.
2. History of the Prior Art
Distillation columns are utilized to separate selected components from a multi-component stream. Generally, such gas-liquid contact columns utilize either cartridge trays, packings or combinations thereof. In recent years the trend has been to replace the so-called "bubble caps" by sieve and valve trays in most trayed column designs, and the popularity of packed columns, either random (dumped) or structured packings has been utilized in combination with the trays in order to effect improved separation of the components in the stream.
Successful fractionation in the column is dependent upon intimate contact between liquid and vapor phases. Some vapor and liquid contact devices, such as trays, are characterized by relatively high pressure drop and relatively high liquid hold-up. Another type of vapor and liquid contact apparatus, namely structured high efficiency packing, has also become popular for certain applications. Such packing is energy efficient because it has low pressure drop and low liquid hold-up. However, these very properties at times make columns equipped with structured packing difficult to operate in a stable, consistent manner. Moreover, many applications simply require the use of trays.
When cartridge trays are the predominant column contacting devices, there is little need to be concerned about vapor distribution because pressure drop across the trayed column is high. For trayed towers with approximately 50 trays, a pressure drop on the order of 6 PSI (300 mmHg) is common in the prior art. This is, however, more than an order of magnitude greater than the kinetic energy generated by the incoming vapor. The velocity head of vapor entering the distillation column is often greater than 3 to 4 inches of water in refinery heavy oil fractionators whereas the velocity head is no more than 5 mm in chemical or gas treating columns. It is true, however, that when the trays of a 50 tray tower are replaced by packing, the pressure drop through the column is typically reduced by a full order of magnitude, to with on the order of 30 mmHg. This is especially true of structured packing such as that set forth and described in U.S. Pat. No. 4,604,247 assigned to the assignee of the present invention. If the kinetic energy of the feed vapor is kept at 10 mm or more, severe mal-distribution may occur.
Cartridge trays or distillation trays may be assembled in bundles of up to 15 or more trays in a tower. The bundles are inserted into the towers ranging in size from 6" to 36" in diameter. To seal the trays against vapor and liquid bypass, cartridge trays are designed with an edge seal on each tray. This sealing technique is extremely important in the design and operation of the tower. The basic theory of a distillation tray is to maintain a liquid level on the tray and allow the vapor to pass through an open area on the tray and through the liquid disposed thereabove. At the same time, the liquid is allowed to flow across the tray into a downcomer for passage to the tray below. This action is controlled by the design of the hole area in the tray floor. Hole area is calculated to maintain a certain vapor velocity and to achieve the proper vapor-liquid interaction. However, if leakage of liquid or bypassing of vapor occurs at the edge of the tray, the design conditions will be altered and the tray will not operate in accordance with the specification. For this reason the design and manufacture of the trays has received considerable attention.
The fabrication and assembly of cartridge tray tower shells are subject to a number of tolerance problems. For example, shells that are fabricated from rolled and welded plates formed into a round shell are affeoted by the heat of welding. When rolled and welded methods of fabrication are used, shell size and tolerance variations occur in the diameter and velocity at the shell flanges and nozzle locations as well as along the axial and circumferntial weld seams. In addition to the problems recited above, the irregularities in the shell surface may restrict movements of prior art tray sealing designs making it difficult or impossible to insert a tray bundle into a tower region where the deformation has occurred
A particularly effective cartridge tray design for process columns is the sieve tray. This tray is constructed with a large number of elongate apertures formed in the bottom surface. The apertures permit the ascending vapor to flow into direct engagement with the liquid that is flowing across the tray. When there is sufficient vapor flow upwardly through the tray, the liquid is prevented from running downwardly through the apertures (referred to as "weeping"). A small degree of weeping is normal in trays, while a larger degree of weeping is detrimental to the capacity and efficiency of a tray. A further discussion of cartridge trays and related aspects of process column operations may also be seen in U.S. Pat. No. 4,956,127, assigned to the assignee of the present invention.
In the assembly stage, the cartridge trays are generally placed in the process column atop support rings. The support rings are generally welded or otherwise permanently secured to the inside surface of the tower and provide mechanical support for the cartridge tray. The issue of sealing the cartridge tray to the column walls and/or the underlying support ring is always a consideration. Prior art approaches have included expandable metal rings which engage the cylindrical walls of the process column. However, out-of-round problems as well as manufacturing tolerance variations often prevent a uniform sealing therearound. It has also been observed that welding along the tower wall in conjunction with construction of tower internals often causes thermal deformation of the tower walls which further exacerbates the out-of-round condition. The utilization of inflexible sealing members against such a tower wall thus generates a myriad of sealing problems. To accommodate for out-of-round regions, more flexible sealing members have been proposed. In the main, the sealing members comprise gaskets and the like which elastically deform to accommodate shape variations. Unfortunately many of the materials for which the flexible gaskets are fabricated find the tower environment to be extremely hostile, and gasket deterioration is commonplace. For this reason, improved sealing member designs have included rings which have a higher degree of flexibility and many accommodate hostile environments. One such ring is set forth and shown in U.S. Pat. No. 4,255,363 wherein a ring is made from polytetrafluoroethylene (PTFE). In that prior art reference it is seen that the PTFE ring is assembled with means for adjusting the pressure of the seal against the tower wall and to accommodate for more effective sealing. Yet even the degree of flexibility afforded by PTFE or other synthetic fluorine material, in and of itself, may not always be sufficient for certain tolerance variations in the tower wall that could be accommodated by more flaccid structures. It would be an advantage, therefore, to overcome the problems of the prior art by providing a reliable, flexible seal that could accommodate both wise tolerance variations and the hostile environment of a chemical process tower.
The present invention provides such an advancement over the prior art by utilizing a teflon impregnated fiberglass gasket of generally flexible construction. The gasket is secured to the cartridge tray ring and presents a double lip outwardly thereof adapted for engaging the process tower wall. The lips are presented in an upwardly deflected orientation for providing a flexible sealing surface against the tower wall and permitting the build-up of liquid pressure thereagainst while preventing the passage of liquid and vapor therethrough and around the cartridge tray perimeter.